Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etching, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Measurement processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. As design rules and process windows continue to shrink in size, metrology systems are required to capture a wider range of physical defects on wafer surfaces while maintaining high throughput.
Modern, complex optical metrology systems are characterized by multiple sets of system parameters such as a range of polar angles of incidence (AOI), a range of azimuth angles of incidence, a range of illumination wavelengths, a range of polarization states, a range of diffraction orders, etc.
Traditionally, measurement data is collected over the full range of each of these multiple sets of system parameters to maximize the amount of measurement data available for analysis to meet the semiconductor device metrology challenge. Often, a significant portion of the data acquisition is performed sequentially. However, this approach to data collection and analysis is often prohibitively time consuming.
In one example, it is contemplated that a two dimensional detector (e.g., a charge coupled device camera) is employed to resolve two beam properties. State of the art two dimensional detectors (e.g., back-thinned CCD detectors) allow for a two dimensional readout of the active area by shifting pixels vertically to a shift register at the bottom of the active area, followed by reading out each pixel of the shift register. This mode reads one row at a time and requires M×N clock cycles to read the active area, where M is the number of columns of pixels and N is the number of rows of pixels. In one example, each column of pixels extends in a vertical direction and each row of pixels extends in a horizontal direction. This mode takes on the order of 100 to 200 milliseconds for an array of 1024×38 pixels. This results in an intolerably slow mode of operation. In addition, if the detector is exposed to light during readout, this leads to mixing of information for a signal dispersed across a number of rows of pixels.
Accordingly, in a conventional spectroscopic system, where charges are being accumulated continuously, one may resolve a spectrum along the vertical dimension, but not without introducing some systematic error and/or giving up a significant amount of useful photon flux. In one example, different angles of incidence (AOI) are dispersed along the vertical direction. Information accumulated from one portion of the pupil at a given row is mixed with the information from an adjacent portion of the pupil for each vertical shift. This process repeats for all rows of exposed pixels. The beam could be shuttered during the reading of the CCD. However, conventional shutters have too much inertia to follow at a rate that corresponds to integration times on the order of two milliseconds. As a result, throughput would suffer.
In another example, a two dimensional CCD is operated in a vertically binned mode. The charges from all rows are first transferred to a shift register at the bottom of the active area. Subsequently, the shift register is clocked out once for the given number of pixels in the longitudinal direction. Such an operation takes M+N clock cycles, and thus is significantly faster than a full 2D-readout (i.e., M×N). However, the resolution in the vertical dimension is lost. Thus, vertical binning is a one dimensional mode of operation for a two dimensional detector.
Improvements to the dynamic range and throughput of array based detectors employed in optical metrology systems are desired to detect a comprehensive set of information from the specimen, including, but not limited to, surface and substrate (bulk) properties, defects, film thicknesses, critical dimensions (CD), composition (n & k), anisotropy, scattered light/surface roughness, edge roughness, etc., on a wafer with greater sensitivity using multiple sets of system parameters at sufficiently high throughput.